Polyethylene reaction rate

A PP sample after ozonization in the presence of UV-irradiation becomes brittle after 8 hrs of exposure, whereas the same effect in ozone is noticeable after 50-60 hours.Degradation of polymer chain occurs as a result of decomposition of peroxy radicals. The oxidation rapidly reaches saturation, suggesting the surface nature of ozone and atomic oxygen against of PP as a consequence of limited diffusion of both oxygen species into the polymer. Ozone reacts with PP mainly on the surface since the reaction rate and the concentration of intermediate peroxy radicals are proportional to the surface area and not the weight of the polymer. It has been found that polyethylene is attacked only to a depth of 5-7 microns (45). 

Figure 5.4 Effect of gas flow rate on (a) the SSP reaction rate of PET at temperatures of 190 and 220 °C, and (b) the rate of increase of the intrinsic viscosity of PET at various temperatures [13]. Reprinted from Polymer, 39, Huang, B. and Walsh, J. J., Solid-phase polymerization mechanism of polyethylene tereph-thalate) affected by gas flow velocity and particle size, 6991-6999, Copyright (1998), with permission from Elsevier Science...

The influence of the ratio of hydroxylic/carboxylic end groups has been studied by several research groups. In the case of PET, this varies, based on the assumed mechanism over the range of 1.5-4.5 1. For poly(butylene terephthalate) (PBT) and polyethylene naphthalate) (PEN), the optimum is indicated at 2.0 1 [19, 20]. Any deviation from this ratio affects the reaction rate. 

Polyethylene glycol has been used as reducing and stabilizing agent for Au NPs. The stability of the resulting Au colloids and the reaction rates are dependent on polymer molar mass. The Au NPs are characterized using UV-Vis, analyzing the plasmon bands [108]. 

Many industrially important chemical processes are high pressure processes. Examples are the production of ammonia and the production of low density polyethylene. Basically, the pressure affects both the equilibrium yield of a chemical reaction and the reaction rate. Here, only the influence on the equilibrium yield is discussed. 

A similar technique is applied to low-density polyethylene reactors. Some of these systems operate in cooled tubular reactors at a very high pressure. Since the reactor has a thick tube wall, the temperature response to changes in the coolant is slow. Instead, the reaction rate (and thereby temperature.) is controlled by injecting initiator at select places along the length of the reactor tube (see Fig. 4.28). 

These two complexes were also found to be active photocatalysts for the polymerization of ethylene (77, 47, 47a). Irradiation of solutions of either complex under 1 atm ethylene resulted in the rapid formation of high-molecular-weight polyethylene. The rate of ethylene polymerization was increased by the addition of various metal halides prior to photolysis. The mechanism of this reaction was not investigated but the authors postulated that the active species were photogenerated Ziegler-Natta-type catalysts (77). 

Equation (9.3) was used to model the thermocatalytic degradation of waste polyethylene and polypropylene [22]. In this case researchers had to calculate p for each catalyst. On the other hand it is complicated therefore researchers disregard the change of reaction rate and order caused by deactivation of catalysts in most experiments. The reaction rates and other reaction kinetic parameters are given in Table 9.2 [32]. 

Polyethylene glycol in the synthesis of materials. PEG has been used as a solvent in polymerization reactions. It was found to facilitate easy removal of the metal catalyst in transition metal mediated living radical polymerization (Figure 8.10). Products from this type of polymerization are usually heavily contaminated with intensely coloured copper impurities. In the case of methyl methacrylate polymerization the reaction rate was higher than in conventional organic solvents, but for styrene the reaction was slower than in xylene. 

NADH may be very high, but then the reaction should be started with a very small amount of NADH and the reaction rate would be very low. A sufficiently high rate is obtained if the concentration of NAD(H) is around its Kj but at those concentrations it will be necessary to recover it after the reaction. A solution to this problem is the enzyme membrane reactor (Fig. 7.4). This is a kind of CSTR (continuous stirred tank reactor) that retains the enzyme and the cofactor using an ultrafiltration membrane. This membrane has a molecular weight cut-off of about 10,000. Enzymes usually have a molecular mass of 25,000-250,000, but the molecular mass of NAD(H) is much too low for retention. Hence, it is first derivatized with polyethylene glycol (PEG 20,000). The reactivity of NAD(H) is usually not greatly affected by the derivatization with this soluble polymer. As a result, alanine can now be produced continuously by high concentrations of both enzymes and of NAD(H) in this reactor. 

Modification of the surface properties by introducing additional hydrophobic residues via mixed carboxylic acid anhydrides of fatty acids and oxa derivatives [43] or amphipathic molecules like polyethylene glycol [44] has been generally used to effect the stability or reaction rates of enzymes in non-polar organic media. In some cases the enzyme became insoluble in aqueous systems [43] or soluble and active in organic solvents [45,46]. 

A powder of hpoprotein hpase (LPL) esterified an organic substrate in toluene at a rather poor reaction rate (Table 4), which was to some extent explained by adhesion of the sticky enzyme powder to the surface of the reaction vessel [7]. When polyethylene glycol (PEG) was bound covalently to LPL and this modified enzyme was dissolved in toluene, approximately 3.5 U mg of enzyme protein were assayed. After simple addition of PEG to the reaction mixture together with LPL powder, the same poor reaction rate of the enzyme powder alone was observed. On the other hand, when LPL powder was lyophilized together with PEG the resultant preparation had an activity of 1.8 U mg L In this case, the enzyme... 

Oxidative chain scission processes in polyethylene have been described in detail (6). During the course of exhaustive oxidation, the reaction rate subsides as accessible... 

Addition of monomer to backbone radical. The addition of the monomer to the backbone radical R- may be considered as equivalent to the copolymerization of M2 with the radical from monomer Mj. Thus, in the terminology of the copolymerization equation the reaction rate coefficient of importance is ki2 and the estimates of the magnitude of this place it sufficiently high to ensure that the monomer will add to the mdical site in polyethylene (Russell, 2002). The monomer will add preferentially to tertiary sites over secondary sites in ethylene copolymers with a high concentration of propylene, but in LDPE there seems to be little preference for branch points. 

Hydrogen abstraction is known to occur from secondary carbon atoms in polyethylene (12) and may also occur in polypropylene, but with lower reaction rates. For polypropylene it was shown that intramolecular hydrogen abstraction in a six-ring favourable stereochemical arrangement will preferentially lead to the formation of sequences of hydroperoxides in close proximity (Scheirs et al, 1995b, Chien et al, 1968, Mayo, 1978). Infrared studies of polypropylene hydroperoxides showed that more than 90% of these groups were intramolecularly... 

A simple method to measure the membrane permeability to specific molecules has been presented by G. Battaglia and coworkers [141], The authors encapsulated highly hydrophilic 3,3, 3//-phosphinidynetris-benzenesulfonic acid (PH) into polyethylene oxidc)-co-poly(butylene oxide) (EB) vesicles and monitored its reaction with 5,5/-dithiobis-2-nitrobenzoic acid (DTNB) penetrating the membrane from the exterior. The reaction rate (amount of the formed product as a function of time after DTNB addition) measured with IJV/Vis was directly correlated to the permeability of the permeating molecule. A comparison of these results with the permeability of egg yolk phosphatidylcholine (PC) vesicles showed that EB membranes have a more selective permeability toward polar molecules than the phospholipids membranes. Also in this case the permeability appeared to depend on the membrane thickness as predicted by Fick s first law. 

The reaction rate is typically regarded as approximately first order in ethylene in polyethylene manufacture with the Phillips catalyst [47,52,349, 379,560,637,727-729]. In the solution process, this relationship holds well, at least when normal commercial concentrations are encountered. However, in slurry or fluidized-bed polymerization, at lower temperatures when the induction time can be significant, the dependence of rate on the ethylene concentration becomes more complex. This complication results because the initiation reactions, reduction and alkylation, also show a strong dependence on ethylene concentration, in addition to the polymerization itself. As noted above for Cr/AIPO4 (Figures 171 and 172), these initiation reactions can exhibit higher reaction orders than first [637]. 

As it was mentioned above, polypropylenes are more prone to oxidation, hence, requiring significantly higher amounts of antioxidants and UV stabilizers compared to PE. It was shown that oxygen intake is much faster in polypropylene compared to that in PE [10], The primary reason is in the microbranched chemical structure of PP (see above), containing tertiary hydrogens that makes formation of hydroperoxides in PP much easier compared to that in polyethylenes. Overall, the mechanisms of oxidation (both photo- and thermooxidation) in PP and PE are quite different. For example, the termination reaction rates for oxidation in PE are 100-1000 times faster compared to PP [11]. 

Consistent with the above observations is the recently proposed order of the intermolecular reaction rates in polyethylene oxide amorphous > interfacial > crystalline [Zhang et al., 1992]. The role of oxygen diffusion during the radiation-induced degradation was also brought out by the work of Burillo et al. [1992], who found that, because of the reduced diffusion of oxygen, the extent of oxidative reactions was lower in compressed PVC samples (P < 880 MPa) than in uncompressed ones. 

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